Oxidation resistant high temperature structures



Nov. 3, 1970 1-, w m ETAL 3,538,231

OXIDATION RESISTANT HIGH TEMPERATURE STRUCTURES Filed March 25, 1969 2Sheets-Sheet 1 FIG. I

1 l2 l4\ I I 7' 1 POWER SUPPLY IOO FIG. 2

A I00 90 7O 6O 5O 4O 3O 20 IO WEIGHT PERCENT INVENTORS TERRY F. NEWKIRKBY MARC S.NEWKIRK Qw/u'sdf.

AT TOR NEYS NOV. 3, 1970 1', NEWKlRK ETAL 3,538,231

' OXIDATION RESISTANT HIGH TEMPERATURE STRUCTURES Filed March 25, 1969Sheets-Sheet 2 FIG. 4

'INVENTORS TERRY F. NEWKIRK MARC S. NEWKIRK 4 i ATTORNEYS United StatesPatent O 3,538,231 OXIDATION RESISTANT HIGH TEMPERATURE STRUCTURES TerryF. Newkirk and Marc S. Newkirk, Lynnfield, Mass, assignors toInternational Materials, Lynnfield, Mass. Filed Mar. 25,1969, Ser. No.810,318 Int. Cl. Hb 3/66 U.S. CI. 13-25 24 Claims ABSTRACT OF THEDISCLOSURE A structure such as a crucible or heating element whichoperates at very high temperatures in air without atmospheric corrosionhas a core member made of tungsten, graphite, carbon or molybdenum. Aprotective coating covers the core member and is composed of aluminum,preferably also tungsten and a low-melting, nonreactive metal from thegroup consisting of indium, tin, and gallium.

At the operating temperatures of the structure, the coating remains in apartially liquid state forming a continuous protective film on the coremember for an extended period.

BACKGROUND OF THE INVENTION This invention relates to structures whichare capable of operating at very high temperatures without degradationdue to oxidation. While we are concerned here specifically with hightemperature electrical heating elements, the invention has applicationalso in connection with crucibles, heat shields and linings, furnacecomponents, heated filaments and other such structures which encountervery high temperatures in normal use.

Conventionally, heating elements are made of tungsten, carbon, graphite,molybdenum and other such materials which are able to withstandrelatively high temperatures. However, at very high temperatures, e.g.over 1300" C., these materials deteriorate very rapidly due tooxidation. As a result, the prior elements burn out almost immediately.Even at lower temperatures, these elements deteriorate, although at aslower pace, so that their heat outputs vary and they still have arelatively short life.

In order to minimize the degradation due to atmospheric corrosion ofstructures made of these refractory materials, the elements are usuallyoperated in an inert atmosphere. However, this requires that they behoused in an airtight enclosure which has to be purged of air or chargedwith an inert gas each time work is placed in the enclosure. This, ofcourse, makes the overall unit relatively large and heavy, as well asexpensive.

Heating elements made of other materials do exist which can be operatedin air for varying periods of time at very high temperatures. However,they have other disadvantages which limit their application. Forexample, structures made of some platinum alloys can operate attemperatures on the order of l600l700 C. However, they are expensive,and their useful lives at these temperatures are quite short. Forexample, a 0.20 inch diameter platinum rhodium alloy element operated ata furnace temperature of 1650 C. will burn out after only 5 or 6 hours.If the temperature is reduced to 1600 C., its life span is still only aday or two. Moreover, this type of element must be very small and beoperated in a small furnace, i.e. a inch diameter tube, in order todevelop these high temperatures without prohibitive cost. Therefore, theamount of work that can be heated by it at any one time is very small.

Silicon carbide rods are capable of operating at temperatures as high as1700-1750 C. Here again, however, their useful lives are quite short. Inaddition, the silicon carbide material is quite brittle and has a lowtensile strength. Therefore, the element as a whole is quite fragile.Another disadvantage of the silicon carbide rods is that, at hightemperatures, they have a very low watt density with the result that ittakes a relatively long time for the heating elements to reach theiroperating temperatures.

Still other conventional high temperature heating elements have beenmade of molybdenum disilicide. These, like the silicon carbide rods, arequite fragile. Moreover, when the elements are oriented horizontally,they tend to sag. Therefore, they need support all along their lengths.Also, deterioration tends to occur at these supported regions and thesupports themselves must be built to withstand the very high operatingtemperatures.

Heating elements constructed of other materials have been proposed.However, invariably they are deficient in one or more of the respects ofhigh cost, fragility and short life.

SUMMARY OF THE INVENTION Accordingly, this invention aims to provide astructure which is capable of operating at very high temperatures in airwith minimal atmospheric corrosion.

Another object of the invention is to provide an improved electricalheating element which can operate at these high temperatures for arelatively long period without failure.

A further object of the invention is to provide an improved electricalheating element which can withstand repeated temperature cyclings over arelatively wide temperature range.

Another object is to provide a high temperature heating element havingselected reproductible heating characteristics.

Yet another object of the invention is to provide a high temperatureheating element which is supported only at its ends, yet which does notsag in use.

Still another object of the invention is to provide an improved hightemperature electric furnace using one or more of these heatingelements.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the features of construction,combination of elements and arrangement of parts which will beexemplified in the constructions hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

Briefly, our improved high temperature structure comprises a core membermade of tungsten, graphite, carbon, molybdenum or combinations of theserefractory materials. A protective alloy coating covers the core memberand is comprised of aluminum, preferably also tungsten, and alow-melting, nonreactive metal from the group consisting of indium, tin,and gallium. The preferred compositions of the alloy coatings will bedescribed in more detail later.

At room temperature, the alloy coating forms a solid sheath around thecore member. However, at the normal operating temperature range of theheating element or other structure, the coating is maintained in apartially liquid state so that it forms a continuous protective film onthe core member which isolates the latter from the air and therebyprotects it from oxidation and other atmospheric corrosion. Also, forthis reason, the coating is able to tolerate the expansion andcontraction of the core member during temperature changes.

The aluminum component of the coating provides a protective oxide filmon the surface of the element. The indium, tin or gallium form a lowmelting fluid at the operating temperature which is essentiallynonreactive with the underlying core material and which enables thealuminum oxide to remain fiowable on the surface of the core member soas to preserve the continuity of the coating. The tungsten is desirablebecause it lowers the solubility of the coating for the core materialand minimizes attack on it. It also increases the viscosity of the alloyso that a coating of optimum thickness can be applied to the coremember.

Heating elements made in accordance with this invention are able tooperate in air at temperatures as high as 1900 C. and more forrelatively long periods without failure. This is in sharp contrast touncoated elements having the same core material which fail substantiallyimmediately (i.e. within 1-3 seconds). Moreover, the elements can berecycled many times between these high temperatures and room temperaturewithout any material adverse affect. Also, the elements are structurallyrelatively strong and durable and have a minimum tendency to sag.Therefore, they can be supported solely at their ends. Yet, with all ofthese advantages, the elements are still relatively easy and inexpensiveto make compared to those prior conventional ones which are able towithstand these very high temperatures. Consequently, a relatively largevolume, high temperature furnace can be constructed using a number ofthese elements without the cost of the furnace becoming excessivelyhigh.

BRIEF DESCRIPTION OF THE DRAWINGS For a fuller understanding of thenature and objects of the invention, reference should be had to thefollowing detailed description taken in connection with the accompanyingdrawings, in which:

FIG. 1 is an isometric view of a high temperature electric heating unitmade in accordance with this in- 'vention;

FIG. 2 is a triangular compositional diagram of the coating utilized inthe practice of the present invention; A FIG. 3 is an isometric view ofanother embodiment of our heating element; and

FIG. 4 is a sectional view with parts in side elevation showing anotherheating element embodiment used in an electric furnace.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 of thedrawings, our heating element comprises a solid core member 12 made oftungsten, graphite, carbon or molybdenum. Member 12 may be a straightrod or wire, as shown, or it may be formed into a loop or coil dependingupon the particular application. A coating 14, to be described in moredetail later, covers core member 12.

In use, element 10 is heated by passing an electric current through it.For this, a pair of sleeve-like electrodes 16a and 1612 are engaged overthe ends of the element and these are connected by electrical leads 18aand 18b to a standard power supply 20. Electrodes 16a and 16b are madeof a highly thermally conductive metal such as brass and they arerelatively massive so that they can also function as heat sinks to coolthe ends of the heating element in the vicinity of its electricalconnections to power supply 20. With this arrangement, element 10 canoperate in air at precisely controlled very high temperatures, in excessof 1300 C., for a relatively long period without failure. Moreover, itcan be recycled between these high temperatures and room temperaturemany times without appreciably shortening its operating life.

Coating 14 is an alloy composed of aluminum and a metal selected fromthe group consisting of indium, tin and gallium; and a preferred coatingalso includes tungsten.

Referring now to FIG. 2, all possible coating compositions areillustrated by that triangular diagram. However, all compositions(represented by the diagram as a whole) do not make a satisfactorycoating 14. When alloy coating 14 is composed of the ingredients mixedin the proportions shown by the shaded area designated ABC in FIG. 2,then the operating temperature and life of element 10 are substantiallyincreased, with optimum results being obtained with compositions withinthe range indicated by the shaded area DEFG.

Turning again to FIG. 1, when applying coating 14 to core member 1 2,the coating ingredients are melted and thoroughly mixed. Then the coremember is dipped into the molten alloy. Alternatively, the coating maybe sprayed or sputtered directly onto the surface of member 12. At roomtemperature, the alloy coating 14 forms a solid sheath which cladsmember 12. Over most of the operating range of the heating element,however, coating 14 is partially liquid so that it forms a flowable,oxidation-resistant film on the surface of member 12 which protects thelatter from atmospheric corrosion. The fact that the alloy coating isflowable when element 10 is in use insures that the protective film iscontinuous over the entire surface of core member 12. Thus, no part ofthat member is exposed to the atmosphere. Also, the alloy coating isable to follow readily the expansions and contractions of core member 12as that changes temperature so that there is no breach in the alloycoating at any point in the operating cycle.

The precise reason why the coating ingredients coact as they do toenable element 10 to withstand such high temperatures in air is notaltogether understood. It is believed, however, that the aluminum formsa protective oxide film on the surface of the coating. The indium, tinor gallium, on the other hand, provide a low melting fluid medium whichis essentially nonreactive and, when used in conjunction with the othercoating components, remain in equilibrium with the underlying corematerial so that the oxide film tends continuously to heal itself.Aluminum, without indium, tin or gallium, does not possess this healingcapability and does not appreciably improve the operating life of theheating element.

Preferably, coating 14 also includes tungsten. This material increasesthe viscosity of the coating alloy so that a coating of optimumthickness can be applied (i.e. 8-20 mils). In addition, when theunderlying core member 12 is itself made of tungsten, the tungstencomponent in coating 14 minimizes attack on the core member. On theother hand, when member 12 is made of graphite, the tungsten in thecoating serves to enhance the adherence of the coating to the graphite.For these reasons, tungsten improves the oxidation protection affordedmember 12 and thereby raises the operating temperature of element 10 andprolongs its useful life.

It is also found that oxidation protection is maximized if a smallamount of the tungsten in the coating is replaced by boron. Although theboron content of coating 14 can be as high as 10% by weight, thepreferred amount of boron is found to be between 0.5% and 2%, with 1%being the optimum. In any event, the tungsten content of the coatingshould still be at least 5% by weight as shown by area DEFG in FIG. 2.

In some applications, for reasons of cost or availability, gold, silveror copper may be substituted for a minor part of the indium, tin orgallium. Also, for the same reasons, a minor amount of aluminum (i.e. upto 30 wt. percent) may be replaced by chromium.

Tests were run on a number of heating elements with the results notedbelow. In each case, the coating 14 was applied by dipping core member12 in the molten alloy and the coating materials used were of very highpurity. Each element was connected in the arrangement shown in FIG. 1and the temperature of the heating element was raised immediately to theindicated temperature in air as measured by at Leeds and Northrup Ray 0Tube brand radiation pyrometer focused on the center of the element.

EXAMPLE 1 A coating was applied to a 0.125 inch diameter, 4 inch long,tungsten rod to a thickness of approximately 8 mils. The coatingcomposition by weight percent was as follows (see point 1 in FIG. 2):

Percent Tin 60 Aluminum 30 Tungsten 9 Boron 1 The temperature of theheating element was raised immediately to 1820 C. by passing a currentthrough it. The heating element was maintained at this temperature for23 /2 hours in open air before failure. Failure was indicated, as in allthe other tests, by a trail of core material oxide vapor originatingfrom a point on the heating element.

Further, this construction was found to be quite re sistant to thermalcycling. For example, a heating element made in accordance with Example1 was cycled in second intervals from room temperature to 1800 C. anddown again for 50 cycles without failure or flaking of the coating.

EXAMPLE 2 A heating element consisting of a 0.100 diameter tungsten rodcore member was coated with the Example 1 coating material. This elementwas heated to 1900 C. and maintained at that temperature for four hoursin open air before failure.

EXAMPLE 3 The heating element having a tungsten rod core was coated withthe following alloy (see point 1 in FIG. 2):

Percent Tin 60 Aluminum Chromium 10 Tungsten 9 Boron 1 This element washeated gradually while monitoring it with the pyrometer. Failure of thecoating did not occur until a temperature of 2085 C. was attained.

EXAMPLE 4 p A heating element having a 0.175 inch diameter tungsten rodas a core member had a coating of the following composition (see point 4in FIG. 2)

Percent Tin 50 Aluminum 40 Tungsten 9 Boron 1 This element was operatedat 1820 C. for 33 hours in air without failure at the time the test wasterminated.

The following examples show results obtained at various extremes of theFIG. 2 coating composition range.

EXAMPLE 5 A heating element having a 0.100 inch diameter tungsten rod asa core member was coated with an alloy of the following composition (seepoint 5 in FIG. 2):

Percent Tungsten 39 Aluminum Tin 30 Boron 1 The heating element wasraised immediately to 1800 C. and lasted 5 /2 hours before failure. Acomparable uncoated element failed in l-2 seconds.

EXAMPLE 6 A heating element having a tungsten rod core member had analloy coating of the following composition (see point 6 in FIG. 2)

Percent Tin Aluminum 10 Tungsten 9 Boron 1 This element lasted for 2 /2hours at 1800 C. A comparable uncoated element failed immediately.

EXAMPLE 7 A similar tungsten core element had an alloy coating of thefollowing composition (see point 7 in FIG. 2):

Percent Aluminum 60 Tin 30 Tungsten 9 Boron 1 This element operated forfour hours at temperatures ranging from 1840 C. to 1950 C.

EXAMPLE 8 I A heating element consisting of 0.100 inch diameter W-shapedtungsten rod was coated with an alloy of the following composition (seepoint 8 in FIG. 2):

Percent Tin 50 Aluminum 30 Tungsten 10 Boron 10 This element was heatedto 1750 C. in air. However, when the temperature was raised to 1800 C.,the coating began to fail.

We should emphasize again at this point that heating elements consistingof equivalent uncoated core members burn out in seconds when operated inair at these high temperatures. Failure is immediately apparent by thepresence of smoke issuing from a point on the heating element indicatingthe formation of an oxide of the core material.

Referring now to FIG. 3, we have found that the operatingcharacteristics of our heating element are enhanced when each end of theelements core member is encased in a graphite end piece before applyingthe protective coating so that the entire unit, including the graphiteend pieces, is coated.

A heating element of this type is indicated generally at 24 in FIG. 3.The element has a solid core member 26 similar to member 12 in FIG. 1. Apair of larger diameter cylindrical graphite end pieces 28a and 2812have axial bores 30a and 30b which are arranged to snugly receive theends of rod 26. Then an alloy coating 32 having a composition defined bythe shaded areas in FIG. 2 is applied to member 26 as well as to thegraphite end pieces 28a and 28b.

Element 24 is heated by passing an electrical current through it by wayof electrodes 34a and 34b which engage over end pieces 28a and 28b.These electrodes also act as heat sinks. The electrodes are normallypositioned so that the portions of end pieces 28a and 28b adjacent rod26 project out from the electrodes about /2 to inch. Actually, onlythese projecting portions need be coated because the remainders of theend pieces within the electrodes (heat sinks) are kept sufiiciently coolthat they suffer no oxidation.

We have found that if end pieces 28a and 28b project out by this amount,after element 24 has been in operation for only a short time, a doughnutshaped annular protuberance tends to form in coating 32 near each endpiece. These formations are indicated at 36a and 36b in FIG. 3 and areexaggerated for clarity.

At the operating temperatures of the element, these formations 36a and36b remain at yellow heat. On the other hand, the projecting portions ofgraphite end pieces 28a and 28b, as well as the portions of the coatedrod 26 immediately adjacent these formations, remain black. Thisindicates that these portions stay relatively cool and we have foundthat this has a beneficial effect on the operating life of element 24 asa whole. I

It is believed that these formations are due initially to thedifferential expansion of the molten alloy coating 32 and tungsten coremember 26, together with the restrictive effect on the coating of thegraphite end 'pieces 28a and 28b. Further, tests indicate that there maybe a chemical as well as a physical effect produced by thisconstruction. That is, these formations 36a and 36b have been foundafter testing to be black to metallic grey in color and very hard togrind. While their composition is not known exactly, they have theappearance and characterisics of a metallic-bonded, mixed oxide-carbidematerial.

The following examples will illustrate the efficacy of the FIG. 3construction:

EXAMPLE 9 A heating element consisting of a 0.175 inch diameter tungstenrod and graphite end pieces approximately 1% inch long and A; inch indiameter were coated with the alloy of Example 1. The element wasmounted in brass heat sinks with the graphite ends protruding asdescribed above in connection with FIG. 3. Electric current was passedthrough the element to bring it rapidly to 1820 C. This element remainedoperating at this temperature in air for 44 hours before failure. Anelement made of a similarly coated tungsten rod, but without thegraphite end pieces failed after 23 /2 hours at this temperature.

EXAMPLE Another heating element having a similar tungsten rod core andgraphite end pieces was coated with the Example 4 alloy. This elementoperated in air at 1820" C. for 47 /2 hours without failure at the timethe test was terminated.

Due to the large amount of heat generated near the upper limits of thetemperature capability of our heating element, the electrical contactsto the element may become overheated unless steps are taken to preventthis. This may be done, for example, by cooling electrodes 34a and 34bby circulating water through them.

Actually, in the FIG. 3 element, the graphite end pieces 28a and 28b,being of relatively large diameter, do not generate appreciable heatduring the passage of current through the element as compared with thecoated core member 26. Therefore, they are often cool enough to serve asthe electrical leads for the element where they etxend through thethermal insulation of a furnace wall.

FIG. 4 illustrates another heating element embodiment indicatedgenerally at 40 mounted in a refractory insulating furnace wall 42. Onlyone side of the furnace wall and one end of the element are shown.

Heating element 40 comprises a tungsten rod core member 44 having alarger diameter graphite end piece 46 similar to end piece 28:: in FIG.3. A coating 47 of the type described above covers core member 44 andend piece 46 as described above in connection with FIG. 3.

The graphite end piece 46 is snugly received in a passage 48 in furnacewall 42. The element is positioned in this passage so that the coatedcore member 44, as well as an inch or so of the coated end piece 46,project out of passage 48 into the furnace cavity 50. End piece 46 thussupports the coated rod 44 and serves as an electrical lead within theintermediate temperature zone inside the furnace wall.

The end piece may extend entirely through the wall. Alternatively, asshown in FIG. 4, a rod 52 of thermally conductive metal such as aluminumcan substitute for the this event, the aluminum rod has a reduceddiameter threaded end portion 52a which screws into a correspondinglythreaded axial bore 46a in end piece 46. The utilization of the aluminumrod 52 is desirable in some cases because the aluminum is less expensiveand stronger than the coated graphite.

A rather massive thermally conductive electrode-heat sink 54 snuglyengages over aluminum rod 52 within wall 42, the passage 48 through thewall being enlarged at 48a to accommodate it. Thus, the graphite and'aluminum members 46 and 52 serve as conductors for the element 40* inthe cooler regions of the furnace, connecting the element bothelectrically and thermally to the heat sink 54.

The same arrangement is, of course, used at the other end of heatingelement 40.

It is important to note that our heating elements are allself-supporting and, unlike some conventional units which operate atmuch lower temperatures, they do not sag appreciably even at theirmaximum service temperature, i.e. 1900 C. As a result, our elements canbe suspended within a furnace chamber solely by their end leads. This ishighly desirable because any contact with the hot midportion of theelement can result in contamination and have a deleterious effect on theelements performance. However, even in those applications where contactwith the element is essential, this can be accomplished using a highpurity alumina ceramic (e.g. 99% A1 0 provided that there is noappreciable abrasion of the coated surface when the element is in use.

It will be seen from the foregoing then that our coated structures areable to function effectively at very high temperatures in air forrelatively long periods without degradation due to oxidation and otheratmospheric corrosion. The high temperature electric heating elementsspecifically described above function effectively in air at worktemperatures on the order of 1900" C. These elements may be used singlyor in groups to maintain a very high operating temperature within arelatively large refractory enclosure. Moreover, since these elementsare capable of withstanding oxidation, there is no need to maintain anoxygen-free atmosphere within the furnace.

The elements described herein are advantaged also in that they arerugged, nonbrittle, durable and have no tendency to buckle or sag inuse. Moreover, they can withstand repeated temperature cyclings overtheir operating ranges without seriously shortening their operatinglives.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in the above constructions Withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

It is also be understood that the following claims are intended to coverall of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

We claim:

1. A structure capable of withstanding very high temperatures in air,said structure comprising (A) a core member made of at least onerefractory material selected from the group consisting of tungsten,graphite, carbon and molybdenum, and

(B) a coating covering said core member, said coating comprising analloy of (1) aluminum, and (2) a low-melting, relatively nonreactivemetal selected from the group consisting of indium, tin and gallium.

2. The structure defined in claim 1 wherein said coating is defined byand included within area A, B, C, of the FIG. 2 diagram.

3. The structure defined in claim 2 wherein a minor amount of saidaluminum is substituted for by chromium.

4. The structure defined in claim 2 wherein said alloy coating alsocontains a minor amount of at least one metal selected from the groupconsisting of gold, silver and copper.

5. The structure defined in claim 2 wherein said alloy coating containstungsten.

6. The structure defined in claim 5 wherein up to by weight of saidtungsten is substituted for by boron.

7. The structure defined in claim 2 wherein said coating is an alloyselected from those defined by and included Within area D, E, F, G ofthe FIG. 2 diagram.

8. The structure defined in claim 7 wherein (A) up to 2% by weight ofsaid tungsten is substituted for by boron, and

(B) said alloy contains a minimum of 5% by weight of tungsten.

9. An improved oxidation-resistant electrical heating element, saidelement comprising (A) an electrically conducting core member composedof at least one material selected from the group consisting of tungsten,graphite, carbon and molybdenum, and

(B) a protective alloy coating covering said core member, said coatingbeing composed of an alloy selected from those defined by and includedwithin area A, B, C of the FIG. 2 diagram.

10. The heating element defined in claim 9 (A) and further including apair of refractory end pieces engaging over the ends of said coremember, and

(B) wherein said coating is applied to said core member and at least thesurface portions of said end pieces adjacent said core member.

11. The heating element defined in claim 10 wherein said end pieces aremade of graphite.

12. The heating element defined in claim 10 and further includingthermally conductive electrodes in intimate contact with said end piecesfor conducting current to the coated core member and conducting heattherefrom.

13. The heating element defined in claim 12 wherein said electrodes arespaced an appreciable distance from the ends of the end pieces adjacentthe coated core member.

14. The heating element defined in claim 12 wherein said end pieces havethermally and electrically conductive metallic extensions.

15. An improved heating element as defined in claim 9 wherein said alloycoating contains tungsten.

16. An improved heating element as defined in claim 15 wherein up to 10%by weight of said tungsten is substituted for by boron.

17. The heating element defined in claim 9 wherein up to 30% by weightof said aluminum is substituted for by chromium.

18. The heating element defined in claim 9 wherein said alloy coatingalso contains a minor amount of at least one metal selected from thegroup consisting of gold, silver and copper.

19. The heating element defined in claim 9 wherein said coating is analloy selected from those defined by and included within area D, E, F, Gof the FIG. 2 diagram.

20. The heating element defined in claim 19 wherein (A) up to 2% byweight of said tungsten is substituted for by boron, and

(B) said coating contains at least 5% by weight of tungsten.

21. An improved high temperature electric furnace comprising (A)thermally insulated container,

(B) one or more electric heating elements mounted in said container forheating its interior, each said element comprising (1) a refractory coremember, and

(2) a coating covering said core member, said coating being composed ofan alloy selected from those defined by and included within the area A,B, C of the FIG. 2 diagram, and

(C) electrically and thermally conductive means supporting the ends ofthe coated core member in the container.

22. The electric furnace defined in claim 21 wherein said coating is analloy selected from those defined by and included Within the area D, E,F, G of the FIG. 2 diagram.

23. The electric furnace defined in claim 22 wherein a minor amount ofthe tungsten in said coating is substituted for by boron.

24. The electric furnace defined in claim 23 wherein said end supportingmeans comprise (A) graphite end pieces engaging over the ends of thecoated core member, the portions of said end pieces adjacent the coremember also having the same coating, and

(B) thermally conductive electrodes engaging around said end pieces atpoints thereon spaced appreciably from the coated core member.

References Cited UNITED STATES PATENTS 7/1962 Whitfield et al. 29198 X6/1967 Scheible 2l9553 X

